1. Field of the Invention
The invention relates to a drier with exhaust gas purification by means of thermal post-combustion for water-damp bulk materials, such as wood chips, green forage and similar organic particles. The dried exhaust gases are purified of organic toxic and odorous substances and also of combustible fine dust by means of thermal post-combustion in the incinerator of the drier.
2. Discussion of the Prior Art
Such a drier operates in a known manner with a vapor feedback line for feeding a portion of the vapor that comes from the drier back to the entrance side of the drying apparatus. The drier thus has a vapor circuit. A surplus amount of vapor, which results from evaporation by the drier as well as from any air that might flow into the vapor circuit from the outside, is removed from the vapor circuit. The drier has its own incinerator, which comprises a combustion chamber with at least one burner for the incineration of gas, oil or combustion dust. The removed vapor flow is supplied to the incinerator as secondary air and is thereby heated to at least c. 800.degree. C. to 850.degree. C., so that the organic toxic and odorous substances contained in the flow are burned to the greatest extent possible.
The drier is not heated directly by the supply of hot incinerator exhaust gases to the drying apparatus. Rather, heat is extracted from the incinerator exhaust gases in a gas-gas heat exchanger and fed to the vapor circuit.
Finally, another heat exchanger is usually arranged in the flow of the incinerator waste gases to serve as an air preheater for the fresh air flowing to the drier, e.g., the combustion air. As a result, the combustion exhaust gases are further cooled before either being supplied to a downstream purification step, e.g., for further dust removal, or released directly into the free air through a exhaust gas chimney.
Drier units of this type are known, for example, from German reference DE OS 40 17 806 and from the product description in "swiss combi news" 1/94 of W. Kunz dryTec AG, CH 5606 Dintikon, Switzerland. Such drier units supply vapor from the drier to the incinerator to bum organic materials, and then feed the incinerator gases through a heat exchanger. The incinerator gases thereby heat the vapor circuit, and thus supply the drier with heat needed to evaporate the water contained in the material being dried. However, known units of this type have the disadvantage that the incinerator gases enter the gas-gas heat exchanger on the heat side at combustion chamber temperature, whereas a temperature of approximately 800.degree. C. to 850.degree. C. is needed to ensure reliable thermal destruction of the toxic and odorous substances. Experience has shown that when the combustion chamber temperature is low, sufficient thermal destruction of the organic toxic and odorous substances does not occur. Moreover, the sulfuric and other substances contained in fine dust and first released in the combustion chamber are insufficiently burned as a result of excessively low temperatures. Indeed, the total amount of toxic and odorous substances can even increase in a combustion chamber given an excessively low temperature.
In previously known driers of this type, the gas-gas heat exchanger alone transfers the total drying heat from the incinerator exhaust gas to the vapor circuit of the drier. Because of the high heat-side temperature load, it is disadvantageously necessary to use massive structures with heat-resistant steels and refractory-grade materials for the heat exchanger, which therefore becomes very large and expensive. It is also disadvantageous that, due to the massive structure required for the heat exchanger, the heat storage mass of the heat exchanger is very high. The control behavior of the drier during operational fluctuations is therefore sluggish. It is also disadvantageous that the gas-gas heat exchanger, given its massive structure and the poor conductivity of its refractory-grade materials compared with sheet metal, has a higher temperature differential from the entrance heat side to the exit cool side and from the entrance cool side to the exit heat side than do exchangers made of sheet metal. Moreover, it is disadvantageous that the gas-gas heat exchanger (which cannot be operated with direct current, because then the heat-side exit temperature would necessarily exceed the cool-side exit temperature), permits maximum temperatures at the cool side exit of only 400.degree. C. to 500.degree. C., so as to avoid damaging the wall between the heat side and the cool side by the temperature load placed upon it. For this reason and because of the poor heat transfer, the gas-gas heat exchanger disadvantageously operates with lower efficiency than sheet metal embodiments. The exit temperature on the heat side is therefore so high that, despite a downstream air pre-heater for inflowing fresh air, the exhaust air temperature at the chimney is still approximately 180.degree. C. This far exceeds the exhaust gas temperatures of 120.degree. C. to 130.degree. C. that are known from directly heated driers for the same bulk materials. It is also disadvantageous that the combustion chamber temperature, which, because of the limited temperature load capacity of the heat exchanger, cannot exceed the required minimum of roughly 800.degree. C. to, at the most, 850.degree. C., is subject to downward fluctuations during operation, resulting in states of reduced toxic substance combustion. Disadvantageously, the exhaust air temperature, which still equals 180.degree. C. after the fresh air preheater, can no longer be used for the drying process. Therefore, this heat, if it is to be used, must be supplied to other users, which is known to be expensive and often is not possible. It is true that arrangements are known, such as that described in the product description "swiss combi news" 1/94 of the W. Kunz dryTec AG, CH 5606, Dintikon, in which the exhaust gases of the drier serve for the direct heating of a pre-drier. Markedly lower exhaust air temperatures are achieved in this case. Disadvantageously, however, not all of the vapor from the pre-drier is fed to the incinerator; thus, some is emitted, along with the toxic and odorous substances contained therein and released in the pre-drier, via the exhaust air without thermal post-combustion.
Accordingly, it is an object of the present invention to provide a drier which, while maintaining at least the same drier efficiency, allows the gas-gas heat exchanger to be designed much more simply and economically, and permits incineration temperatures high enough to reliably bum the toxic and odorous substances contained in the vapors fed to the incinerator, whereby all vapors are fed to the incinerator, and the drier exhaust air temperature can be kept clearly lower than in previously described systems of this type.
Pursuant to this object, and others which will become apparent hereafter, one aspect of the present invention resides in a drier, with a drying apparatus preferably embodied as a rotary drum, having its own incinerator to produce the required drying heat. However, the hot exhaust gases from the incinerator are not fed directly to the drying apparatus for heating purposes. The drier operates in the known manner with vapor feedback, so that the drying apparatus and the feedback for vapors to the drier entrance are part of a vapor circuit. In the vapor circuit, heat is supplied-by means of a gas-gas heat exchanger that removes heat from the incinerator exhaust gases-to the fed-back portion of the vapor flow, which has emerged from the drying apparatus and was cooled during drying, before this fed-back portion of the vapor flow reenters the drying apparatus. Reheated in this manner, the fed-back vapor flow supplies drying heat as a drying medium to the drying apparatus. An excess partial flow that results from evaporation in the drying apparatus and, as applicable, from air flowing into the vapor circuit, is removed from the vapor circuit and fed as secondary air to the incinerator, where the toxic and odorous substances contained in the removed partial flow are burned at the prevailing temperatures. Usually, an air preheater is arranged in the flow of the incineration exhaust gases after the gas-gas heat exchanger. After the incinerator exhaust gases have passed through the gas-gas heat exchanger for the purpose of heating the vapor, the air preheater extracts additional heat from these gases and transmits the heat to fresh air that is supplied to the drier.
According to the invention, an additional heat transfer device is arranged in the flow of the incinerator exhaust gases in front of the gas-gas heat exchanger. The incinerator gases, which enter at combustion chamber temperature on the heat side, flow through the additional heat transfer device. The incinerator gases are thereby cooled, and either produce steam on the cool side or heat a liquid heat-carrier medium of higher volume-specific heat capacity. The drying apparatus is divided, according to the invention, into, firstly, a vapor-heated drying section, where the entering vapors, which were previously heated in the gas-gas heat exchanger, serve as a drying medium while cooling and, secondly, a downstream drying section, where there is at least one heat register to provide additional heat to the drying apparatus. This heat register, while emitting heat on the heat side as a heating medium, condenses steam or cools a liquid heat carrier medium of higher volume-specific heat capacity, as a result of which, in addition to prior heating by vapor, there is drying heat supplied to the drying apparatus. The additional heat transfer device and the heat register arranged in the drying apparatus are connected to each other in a known manner and form a heating medium circuit.
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of the disclosure. For a better understanding of the invention, its operating advantages, and specific objects attained by its use, reference should be had to the drawing and descriptive matter in which there are illustrated and described preferred embodiments of the invention.